The Mark of Satan The upstairs of Maidenhead library was almost certainly used for the library scene. Thanks to Jill Finlay who suggested this was location back in 2006 but whose email I misfiled. I haven't visited the library to make 100% sure the location is correct but the exterior red metal roof structure would seem to confirm it is the right place.
Update March 2019 - Kevin Flanagan thinks that the library may not be in Maidenhead, but Hillingdon, as there is a very similar roofed library there and all the filmining took place in and around Hillingdon. He says that there is also a similar library in Hayes.